Field of the Invention
The present invention relates to an organic light emitting display device. More particularly, the present invention relates to an organic light emitting display device adapted to enhance light efficiency by forming an organic emission layer with separately stacked hole host layers and dopant host layers.
Description of the Related Art
An organic light emitting diode used in an organic light emitting display device is a self-luminous element which includes an emission layer formed between two electrodes. The organic light emitting diode generates excitons by injecting electrons and holes into the emission layer through an electron injection electrode (i.e., a cathode) and a hole injection electrode (i.e., an anode) and recombining the electrons and the holes within the emission layer. Also, the organic light emitting diode emits light when the excitons are transitioned from an excited state into a ground state.
Organic light emitting display devices using organic light emitting diodes are classified into a top-emission mode, a bottom-emission mode and a dual-emission mode according to the light emission directions. Also, organic light emitting display devices can be divided into a passive matrix type and an active matrix type.
In order to display an image, an organic light emitting display device can apply scan signals, data signals and supply voltages to a plurality of sub-pixels, which are arranged in a matrix shape, and enable selected sub-pixels to emit light.
Also, in order to enhance luminous efficiency and color coordination of a display panel, an organic light emitting display device with a micro-cavity structure is being proposed which allows red, green and blue sub-pixels to be formed differently from one another with different thicknesses.
FIG. 1 is a cross-sectional view illustrating electric potential levels for a green sub-pixel of an organic light emitting display device according to the related art. FIG. 2 is a table illustrating characteristics of the green organic light emitting diode shown in FIG. 1.
The organic light emitting diode shown in FIG. 1 corresponds to an organic electronic element which converts electrical energy into light energy. Such an organic light emitting diode includes an organic emission layer EML interposed between an anode electrode E1 and a cathode electrode E2, which is configured to emit light. The anode electrode E1 is used to inject holes, and the cathode electrode E2 is used to inject electrons.
The electrons and the holes injected from the two electrodes E1 and E2 are drifted into the organic emission layer EML and form excitons. The electrical energies of the excitons are converted into visible light so that visible light is emitted. In order to easily and smoothly inject the electrons and the holes from the two electrodes E1 and E2 into the organic emission layer EML, a first hole transport layer HTL and a second hole transport layer G′HTL are formed between the organic emission layer EML and the anode electrode E1, and an electron transport layer ETL and an electron injection layer are formed between the organic emission layer EML and the cathode electrode E2. Moreover, a hole injection layer HIL can be formed between the first hole transport layer HTL and the anode electrode E1.
The organic emission layer EML is formed from a phosphorescent material which is obtained by mixing host materials and a dopant material. The host materials include an E-type host material used to transport the electrons and an H-type host material used to transport the holes. The E-type host material has superior transportation capability compared to the H-type host material. Due to this, the electrons and the holes are not efficiently recombined with each other.
Particularly, light leakage and a fault, such as an emission region shift, can be generated in an interfacial surface between the organic emission layer EML and the second hole transport layer G′HTL.
As shown in FIG. 1, the triplet potential energy T1 of the organic emission layer EML is larger than the triplet potential energy T1 of the electron transport layer. Due to this, energy loss must be generated. As seen from characteristics of an element within the green sup-pixel region which are described in FIG. 2, the current efficiency is about 95 (cd/A) and the current efficiency reduction at the high temperature driving reliability evaluation is about −14%.